The invention relates to a method of heating combustion air entering an internal combustion engine using a heat storage means.
Private car engines have a wide field of application and must satisfy exacting working conditions. On the one hand, engines must start at low ambient temperatures while on the other hand they must produce high power at high rotational speed; for example freeway driving. Normally, 90% or more of the working life of the engines are operated at low speeds and low loads. Three significant operational conditions, cold start-up, low partial load and full load, involve extremely great differences in the combustion air temperature shortly before and during the expansion strokes. The combustion gas temperature, is generally dependent on the instantaneous operating temperature of the engine. Particularly, the dependency is on the temperature of the walls of the combustion chamber, the increase in temperature of the combustion air caused by engine compression, the temperature of the combustion air entering into the combustion chamber and on the combustion air to fuel mass ratio.
During cold start-up and the first minutes of warming up the engine, high fuel consumption and emission peaks occur until a sufficient operating temperature is established. The main cause of this is the increase of additional fuel due to the so-called cold start or warming up enrichment. However, this addition is necessary, in gasoline and diesel engines, in order to achieve a rapid start and heat up of the engine and its fuel burning system, and to overcome the greatly increased frictional losses of the engine. The excess fuel does not combust at a sufficient rate and leads to excessive emission of noxious exhaust gases. In the case of a diesel engine, noise is produced as well. In the warming up phase, catalytic converters for gasoline engines, as for instance three-way converters, are without any significant effect. This is due to the fact that the effect or conversion rate of the converter depends on its operating temperature. For example, a temperature of at least approximately 360.degree. C. is required in a catalytic converter to provide a desired effect. Thus, exhaust gases must reach a correspondingly high temperature to prime the catalytic converter. Thus, high emission peaks occur during cold start-up since the catalytic converter is incapable of providing the desired effect.
Thus, it is desirable to reach the engine's optimum operating temperature as soon as possible after cold start-up. Reaching this temperature will reduce the excess fuel, high exhaust gas emission and, in the case of a diesel, the loud noise associated with cold start-ups.
Possible ways to achieve the above aims which have been investigated so far include the use of a heat storage means to preheat or rapidly heat the engine and catalytic converter. To warm up the engine utilizing coolant and a latent heat storage means, heating power of 100 kW is possible for short intervals to heat the contents of a heat storage means. The contents of the heat storage means may be completely transferred to the engine system within 20 to 30 seconds. In this respect, 1 kW will increase the temperature by 30.degree. to 40.degree. K. This amount of heat, on the one hand is not sufficient to achieve the heating methods theoretically needed maximum, as for instance heating up from -20.degree. C. to +60.degree. C. On the other hand, the required overall size of the heat storage means, with this thermal capacity, will reach its equipment limit which may be reasonably accommodated in a present day automobile. Thus, since it is unlikely that the theoretically maximum of 60.degree. C. will be achieved, there will be significant emission and fuel consumption peaks due to the failure to warm up essential parts of the system. The mixture preparing system of a gasoline engine is for example only influenced to a slight extent by the method and the walls of the combustion chamber, which are so important for the combustion in the engine cylinder, are practically not affected at all. More especially, the optimum combustion gas temperatures are substantially higher than the maximum coolant temperature, as for example, in the region of 1000.degree. C. and higher.
The frequently discussed use of heat storage means in order to heat up the exhaust gas prior to reaching the catalytic converter or in order to heat up the converter itself involves substantial difficulties. The most significant difficulty is the problem in designing the heat storage means to prevent loss of heat while heating up or priming the catalytic converter to temperatures of approximately 360.degree. C. An additional difficulty of heating the catalytic converter by the heat storage means is that overheating of the converter, by the exhaust gases, must be protected against during full load operation.
By way of conclusion one may say that so far neither the required quantities of heat nor the required temperatures have been achieved for an efficient elimination of the problems of fuel consumption and emission peaks at cold start-up.
Further investigation has shown, dependent on the instantaneous state of operation, that heating of the combustion air may be advantageous even when the engine is not at cold start-up or during warm up. The heating of the combustion air may be used to keep changes in the temperature of the combustion gas of the engine in the optimum value range independent of the operational condition so that the fuel consumption and emission values may be improved within the entire operational range. Furthermore, it is possible to reduce the compression ratio to an optimum value during operation, since the high compression needed to achieve the ignition temperature during cold start-up of a diesel engine at low ambient temperatures or, respectively, to achieve such a compression for emission and fuel consumption reasons in the case of a gasoline engine during partial load conditions, may be compensated by heating of the combustion air.
The object of the invention is, in the case of cold starting an internal combustion engine (words missing in German text) in the case of operational conditions the characteristic of the operating temperature is to be kept as close as possible to the optimum value.
The object of the invention is achieved by the release of heat from a heat storage means to the combustion air prior to the entry into the combustion chamber.
One advantage of the method is that the increase in temperature of the combustion air is multiplied by the engine compression and takes effect at a higher temperature level. This higher temperature not only rapidly heats the air processing system but heats the walls of the combustion space because of the high temperature level. In addition, there is an enhancing mechanism with regard to the heat released by the engine compression. This heat is dependent on the polytropic exponent, which increases with the wall temperatures. This ensures that the air in the combustion chamber is able to reach a sufficiently high temperature in a short time. On the basis of experimental work, it is possible to assume that using the heat storage means it is possible to increase the temperature of the combustion air a sufficient value after a few seconds to make possible a drastic reduction of excess fuel during cold start-up and during engine warm up.
From the point of view of process technology the compression temperature is highly significant. An ample compression temperature favors not only ignition in gasoline and diesel engines but also the ensuing course of combustion in the case of cold engines. It is established that soot formation in diesel engines is influenced by such temperatures. A further advantage of heating the combustion air is that the amount of air fed decreases independently of the temperature increase. This means that it is possible at the outset to ensure a high rate of combustion of the amount of fuel used so that correspondingly high exhaust gas temperatures may be attained. The result of this is the emission of exhaust gas from the engine may be drastically lowered but also the following exhaust gas processing means may take effect earlier on.
The compression ratio of the engine may be reduced generally to the same degree that the heat storage means contributes to the increase in temperature of a diesel engine during cold start-up. In the case of a gasoline engine, the heat storage means may contribute to increase the temperature when operating under partial load. Due to reduction of the compression ratio, fuel consumption is improved in the partial load range of a diesel engine. Also, the reduction makes possible higher means pressures in both diesel and gasoline engines. These pressures possibly lead to an increase in torque and power or to an improvement in the power consumption and emission values, in particular, by reduction of the displacement, torque and power are able to remain the same.
It is thus highly advantageous in accordance with the invention for the compression ratio of an internal combustion engine to be lowered to the least desirable ratio. It is a highly advantageous development of the invention to release heat from the storage means to the combustion air so that it is regulated during the entire operating range taking into account a given optimum characteristic of the combustion gas temperature.
It is particularly advantageous for the invention also to be used in conjunction with alternative fuels such as alcohol any rape oil which in part have a high heat of evaporation and in part are very viscous.
In accordance with an advantageous development of the invention the heat storage means is heated via the engine cooling water and when required its heat is released to the combustion air via a water-air heat exchanger. This is the simplest possibility of heating the combustion air by a heat storage means during cold start-up. It is possible to connect known heat storage means for motor vehicles involving charging and discharging, via the engine coolant, with a water-air heat exchanger in such a manner that upon cold start-up the storage means is discharged into this heat exchanger and at the same time the combustion air is heated as it flows through this heat exchanger. At the same time as the engine reaches its operating temperature, the heat storage means is recharged. In this respect an advantageous development of the invention is possible in which the release of heat to the combustion air takes place via the coolant leaving the engine as soon as the temperature of the coolant leaving the engine exceeds the temperature of the coolant leaving the storage means.
A further advantageous feature of the invention is one in which the coolant leaving the engine is caused to flow through the water-air heat exchanger so that heat is available at the latter even after discharge of the storage means. In this respect it is possible for the coolant emerging from the engine to selectively pass through the heat exchanger or to bypass it. Preferably the coolant leaving the engine is caused to pass through the heat storage means as soon as the temperature of the coolant emerging from the heat storage means exceeds the temperature of the coolant leaving the heat storage means.
In order to ensure that during the start-up phase an accelerated heating up of the engine and possibly of the vehicle may take place, the coolant passes, via a water-air heat exchanger, through the vehicle heating system and the heat storage means and may selectively pass into a first circuit via the engine or into a second circuit through an engine bypass. After discharge of the storage means the combustion air may selectively flow through the heat exchanger or bypass it. Thus, in accordance with the operational state of the engine it is possible even in the case of an engine at operating temperature, to heat up the combustion air to influence the compression temperature or, respectively, the combustion gas temperature in order to optimize the fuel consumption and the emission data. In place of this "air regulation" it is possible to have a so-called "water-regulation" inasmuch as the coolant leaving the engine is selectively choked back or caused to pass through the heat exchanger or to bypass it.
In accordance with a further convenient development of the invention the heat storage means is heated by the exhaust gases of the engine. In this respect the heat storage means may be directly or indirectly heated by the exhaust gases of the engine. In accordance with a convenient form of the invention to indirectly heat the storage means, heat from the exhaust gas is released from the exhaust gas to the coolant which passes into the heat storage means, such release being via an exhaust gas-water heat exchanger.
Another convenient development of the invention is during cold start-up the combustion air passes directly through the heat storage means. In this case, the storage means is charged by exhaust gas passing through ducts of the heat storage means in heat exchanging connection with the storage medium, and is discharged by the storage means combustion air passing through the same ducts.
In this case, during start-up of the engine the combustion air first passes through the storage means and is heated. After sufficient heating of the engine, exhaust gas is passed through and charges the storage means. This manner of operation makes it possible to achieve temperature increases of up to 150.degree. K. in the combustion air. This increase is due to heat being transferred directly from the storage medium to the air and to the temperature of the storage means not being limited by the coolant of the engine. As a result storage temperatures of around 200.degree. C. become possible which are able to be coped with by the designer. It is even possible to charge a storage means with the exhaust gases of a diesel engine.
In accordance with a further advantageous development the engine exhaust gases are passed through an exhaust gas-air heat exchanger. The air heated by the exhaust gas air heat exchanger is used to charge the heat storage means and also, during the warm up phase and the partial load range of engine operation, to heat the combustion air. In this respect, discharge of the storage means during cold start-up is performed by direct heat exchange of the combustion air with the storage means. The double utilization of the exhaust gas-air heat exchanger also means that the exhaust gas emission and fuel consumption may be favorably affected even during engine warm up, because the exhaust gas air heat exchanger is able to supply heat without any limit in time.
In order to make the best possible use of the advantages of the invention in accordance with a further preferred development, the change in the temperature of the combustion air is measured and the value thereof is supplied to a regulating device utilized for general regulation of the engine.
The invention also extends to an internal combustion engine to perform the method with an air induction duct and a heated heat storage means. In this case, the air induction duct is provided with a means for selective heat transfer of the air flowing to the engine. In this respect the compression ratio of the engine in the case of a diesel engine, may be reduced generally by the amount to which the heat storage means contributes to increase in the final compression temperature during cold start-up and in the case of a gasoline engine, by the amount which the heat exchanger contributes to the final compression temperature under partial load.
In accordance with an advantageous feature of the invention the air induction duct has two limbs uniting upstream from the opening into the engine. The limbs may be selectively added into the circuit with on of the limbs being provided with a device to transfer heat to the air flowing into the engine.
The device to transfer heat may be in the form of a heat storage means or a heat exchanger which is heated by waste heat of the engine. The heat storage means may be heated by the exhaust gases of the engine. For this purpose the heat storage means may have a duct system connected with the exhaust gas duct and a duct system connected with the air branch duct.
In accordance with a further convenient development the exhaust gas duct has a duct branch containing the heat storage means and a branch bypassing the heat storage means. One of the duct branches is able to be used selectively to carry the exhaust gas, and the branch leading through the heat storage means of the air induction duct is able to be connected selectively counter-currentwise, via the heat storage means, when the exhaust gas duct is connected with the duct branch bypassing the heat storage means.
Another convenient embodiment is one in which the heat storage means and the heat exchanger are arranged jointly with a vehicle heating system in a coolant circuit. The system includes a duct directly connecting the heat storage means with the heat exchanger in such a manner as to bypass the engine. As a result it is possible during cold start-up to transfer stored heat directly to the combustion air in a manner to bypass the engine. Furthermore, the heat exchanger may also be used to heat the combustion air after the engine has heated up. Also, the heat storage means may during cold start-up cold, or in other operational states with a low temperature of the operating means, be used simultaneously or selectively for heating the engine vehicle cab.
In this respect there is the possibility of a further convenient feature in which an exhaust gas-water heat exchanger is placed in the coolant circuit between the coolant exit on the engine and the heat exchanger. The exhaust gas-water heat exchanger is connected with the exhaust gas duct to release heat. The duct is able to selectively bypass the exhaust gas-water heat exchanger. Also, the coolant circuit includes a bypass duct which opens between the exhaust gas-water heat exchanger and the combustion air heat exchanger. Thus, coolant coming from the engine, when there is a substantial heat requirement, may be heated by heat transferred from the exhaust gas to the coolant via the exhaust gas-water heat exchanger before the combustion air takes up heat from the coolant.
A still further convenient form of the invention is one in which the duct branch of the air induction duct with the heat storage means is arranged at a point upstream from the exhaust gas duct. Also, downstream from the heat storage means a return duct branches off from the duct branch of the air induction duct. The return duct opens into the duct branch with the heat storage means. Furthermore a blower is provided in the air circuit containing the exhaust gas-air heat exchanger. The blower is selectively connected with the duct branch containing the heat storage means and at the same time this duct branch is able to be cut off from the duct branch leading directly to the engine so that there is an independent circuit to charge the heat storage means with exhaust gas heat.
In accordance with a convenient feature of the invention the heat exchanger is included in a duct which is acted upon by waste heat of the engine. Also, a bypass duct containing an adjustable choke is positioned in parallel with the first duct.
In the case of a carburetor engine, it is preferred that the device for selectively transferring heat to air flowing to the engine is arranged upstream from the carburetor so that the carburetor carries air which has already been heated.
In order to make optimum use of the advantages of the invention, in accordance with a particularly preferred embodiment, a temperature measuring device is arranged in the air induction duct downstream from the device for selective heat transfer. The output of the device is connected with a device for general regulation of the engine.
In order to ensure that the temperature of the combustion air flowing to the engine is correctly measured in the case of air-regulated systems for heating the combustion air, the temperature measuring device is arranged directly downstream from the joint or union between the two branches of the air induction duct. In the case of exhaust gas or water regulated systems the temperature measuring device is arranged directly downstream from the heat supplying device.
The transfer of the heat to the combustion air is preferably performed as near as possible to the air inlet of the engine. However, due to design reasons it is usually not possible to arrange the heat storage means in this proximity as well. Therefore, at cold start-up, the combustion air is heated with a certain delay dependent upon the length of the duct path between the heat storage means and the heat exchanger. Thus a further advantageous design is to directly position an auxiliary heat storage means upstream from the heat exchanger. In this respect, a small capacity heat storage means with a correspondingly small volume, which does not take up an excessive amount of space, heats the combustion air until the heat from the main heat storage means, which is further removed, is available.
The heat storage means may be a water heat storage means or a latent heat storage means. While a water heat storage means makes possible a very high heating rate for a short time, the latent heat storage means offers a more even heating effect for a longer period of time and all in all substantially more energy per unit volume. In order to receive the advantages of both types of heat storage means, it is particularly convenient if the heat storage means is provided with two sections in the path of the heat transfer medium so that one of the sections is a latent heat storage means and the other is a water heat storage means.
Preferably the auxiliary heat storage means is in the form of a water heat storage means.
In the case of the internal combustion engine with a heat exchanger arranged in the coolant circuit, there is an advantageous design in which the heat storage means is placed upstream and in series with the heat exchanger to effectively charge the heat storage means.
Inasmuch as in the above explanations there are references to cooling water, water heat storage means or for instance a water-air heat exchanger, such language is used by way of simplification without there being any intent to limit the text to water as a medium and in place of water it is possible to have any one suitable coolant as for instance a mixture of water and anti-freeze fluid.
Working embodiments of the invention will now be described in detail with reference to the drawing the following specification.